1. Field of the Invention
The present invention relates to a structure of an optical disc recording medium and a method of manufacturing an optical disc.
2. Description of the Related Art
In recent years, the recording density and capacity of optical discs, which are one type of optical information recording media, have been increased with the increasingly widespread use of personal computers, the introduction and increasingly widespread use of digital terrestrial broadcasting, and accelerated spread of high-definition televisions to ordinary homes. There have been provided optical disc recording media that are capable of recording a larger amount of information, for example, compact discs (CDs), digital versatile discs (DVDs), and Blu-ray Discs (Registered Trademark). Even now, further increases in the recording density and capacity are desired.
A Blu-ray Disc, which is a high-capacity optical disc recording medium, is an optical disc having a diameter of about 12 cm and a thickness of about 1.2 mm. In such a Blu-ray Disc, a recording layer is formed by laminating, in the thickness direction, a metal thin film, a dielectric film, etc. on a surface of a substrate with a thickness of about 1.1 mm, the surface having irregularities. A cover layer with a thickness of about 0.1 mm is provided thereon. Such a Blu-ray Disc has a recording capacity of about 25 GB (gigabytes).
In order to further increase the capacity, multilayer discs including a plurality of recording layers have been developed. A method of manufacturing a multilayer Blu-ray Disc is, for example, described in Japanese Unexamined Patent Application Publication No. 2003-22586. In general, such a disc is manufactured by a method described below.
As an example, a method of manufacturing a two-layer disc including two recording layers will be described below. First, a first recording layer L0 is formed by forming a metal thin film or by depositing a thermally recordable, thin film material or the like on a surface of a resin-molded substrate (disc substrate) with a thickness of about 1.1 mm, the surface having irregularities composed of pits and a groove. Next, a spacer layer with a thickness of several microns to several tens of microns is formed on the recording layer on the substrate, the spacer layer separating the two recording layers. Next, a stamper which has a surface having irregularities composed of pits and a groove is pressed onto the spacer layer so that the pits and the groove are transferred onto the spacer layer. Next, a second recording layer L1 is formed by forming a metal thin film having a predetermined transmittance with respect to the wavelength of the recording/reproducing laser beam or by depositing a thermally recordable, thin film material on the pits and groove transferred onto the spacer layer. Then, a protective layer (cover layer) which protects the second recording layer L1 is formed on the second recording layer L1.
In the case where the number of recording layers is increased to three or more, for example, as described in Japanese Unexamined Patent Application Publication No. 2007-257759, a method is used, in which while taking into consideration interlayer crosstalk during recording and reproducing of signals, the step of forming the second recording layer L1 is repeated several times, thereby, for example, laminating a third recording layer L2 and a fourth recording layer L3 in that order.
In the multilayer Blu-ray Disc thus manufactured, in order to reduce the influence of signal degradation due to the inclination of the disc, it is necessary to form the laminated recording layers within a thickness range of 0.1 mm. In the two-layer disc described above, the second recording layer L1 is formed on the first recording layer L0 through a spacer layer with a thickness of several microns to several tens of microns, and a light-transmissive protective layer is formed thereon.
Furthermore, in the multilayer Blu-ray Disc, in order to ensure homogeneous recording and reading from the innermost periphery toward the outermost periphery of the substrate, the spacer layer provided between the first recording layer L0 and the second recording layer L1 is desired to have homogeneity and uniformity of optical properties and physical dimensions. The protective layer (cover layer) continuously formed on the upper surface of the second recording layer L1 is also desired to have homogeneity and uniformity of optical properties and physical dimensions. At the same time, there is a demand for an inexpensive method of manufacturing media. Furthermore, since one of the major intended uses of such optical disc recording media is long-term storage, it is expected that recorded information is retained for a long period of time. Therefore, there is a demand for a multilayer structure which can have good recording characteristics even in a high temperature, high humidity environment or in a low temperature environment.
As described above, a cover layer composed of a light-transmissive resin layer is provided as a protective layer for the recording layer on the surface side (laser incident surface side) of one of a plurality of recording layers. A spin coating method has been used as a method for applying a light-transmissive resin to a predetermined area.
In the spin coating method, formation of a uniform coating film is a major challenge, and various techniques have been disclosed. For example, Japanese Unexamined Patent Application Publication No. 8-164355 proposes a method in which a uniform film thickness is achieved by producing a uniform airflow during high speed rotation, and Japanese Unexamined Patent Application Publication Nos. 9-173946, 2002-153800, and 2006-95452 each propose a method in which in order to improve the nonuniformity of the coating thickness in the central portion of the substrate, by inclining the axis of rotation, gravity is also used in addition to centrifugal force. Furthermore, Japanese Unexamined Patent Application Publication No. 2008-21353 discloses a technique in which a cover layer is composed of a first layer formed by spin coating and a second layer formed by spin coating, and the thickness of the layer including the two layers is made uniform.